Lithocule literally means tiny stone; in this case, a tiny building block. It is not clear how they maneuver.

In the book, uncounted numbers of these tiny parts are coming together in ocean shallows to create a new island.

...each lithocule knew exactly where it was supposed to go and what it was supposed to do. They were tetrahedral building blocks of calcium and carbon, the size of poppyseeds, each equipped with a power source, a brain and a navigational system.

This is an interesting way to present a stereolithograph technology; in fact, it inverts the current technology completely.

In standard stereolithography, the "smarts" are in the machine that has the design and control over where the particles settle to create the object. The polymer in the tank is "dumb" - it just deposits a solid bit when "told" to by the laser.